In high temperature electrolyzers, electrolysis of water at high temperature is carried out from vaporized water. The function of a high temperature electrolyzer is to transform steam into hydrogen and oxygen according to the following reaction: 2H2O(g)→2H2+O2.
This reaction is conducted via an electrochemical route in the cells of the electrolyzer.
Each elementary cell consists, as this is shown in FIG. 1, of two electrodes, i.e. an anode (1) and a cathode (2), sandwiching a solid electrolyte generally in the form of a membrane (3).
Both electrodes (1, 2) are electron conductors, and the electrolyte (3) is an ion conductor.
The electrochemical reactions occur at the interface between each of the electron conductors and the ion conductor.
At the cathode (2), the half-reaction is the following: 2H2O+4e−→2H2+2O2−;
And at the anode (1), the half-reaction is the following: 2O2−→O2+4e−.
The electrolyte (3), placed between both electrodes, is the migration site of the O2− ions (4) under the effect of the electric field generated by the potential difference imposed between the anode (1) and the cathode (2).
An elementary reactor, illustrated in FIG. 2, consists of an elementary cell (5) as described above, with an anode (1), an electrolyte (3), and a cathode (2) and of two monopolar connectors or more exactly two half-interconnectors (6, 7) which provide electric, hydraulic and thermal functions. This elementary reactor is called a module.
In order to increase the produced hydrogen and oxygen flow rates, and as this is shown in FIG. 3, several elementary modules are stacked (8), the cells (5) then being separated by interconnectors or bipolar interconnection plates (9).
The assembly of the modules (8) is positioned between two upper (10) and lower (11) interconnection plates which bear electric power supplies and gas supplies (12). This is then referred to as a stack (FIG. 3).
There exist two concepts, configurations, architectures for the stacks:                tubular stacks, in which the cells are tubes, and        planar stacks, in which the cells are made as plates like in FIG. 3.        
In the planar architecture, the cells and the interconnectors are in contact in many points. The manufacturing of the stack is subject to fine tolerances as to the flatness of the cells in order to avoid too high contact pressures and an inhomogeneous distribution of the stresses, which may lead to cracking of the cells.
The seal gaskets in a stack have the purposes of preventing a hydrogen leak from the cathode to the neighboring anodes, of preventing an oxygen leak from the anode to the neighboring cathodes, of preventing a hydrogen leak towards the outside of the stack, and finally of limiting the steam leaks from the cathodes towards the anodes.
Within the scope of the development of a stack for high temperature electrolysis (HTE), and as this is shown in FIG. 4, gas-proof gaskets (13) are thus made between the planar electrolysis cells (5) (this positioning, this configuration of the gaskets, seals is only an example among many positionings and configurations of the gaskets), each consisting of an anode/electrolyte/cathode ceramic trilayer, and the metal interconnectors or interconnection plates (9).
It should be noted that the dimensions in μm given in FIG. 4 are only given as examples.
More specifically, a gasket is made between the lower surface of each cell (5) and the upper half-interconnector (14) of the interconnection plate located below the cell, on the one hand, and between the upper surface of each cell and the lower half-interconnector (15) of the interconnection plate located above the cell (5) on the other hand.
These gaskets (13) should generally have a leak rate in air of less than 10−3 Pa·m3·s−1 between 700° C. and 900° C. under a pressure difference from 20 to 500 mbars.
In addition to this seal function, the gasket may, in certain cases, have secondary assembling and electric conduction functions. In other cases, notably in the case of HTEs, it is rather required that the gasket be not an electric conductor. For certain stack architectures, a ceramic part, called a cell support, may be placed between the cells and the interconnectors; and gas-proof gaskets are then also required with this cell-supporting part.
It should be noted that the description which is made above of the stacks as well as of the arrangement of the gaskets is only given as an example. There exist many architectures for these stacks and there also exist many configurations for the gaskets which we shall not detail here for the sake of simplification.
The present invention is of general application regardless of the nature, the geometry, the architecture of the stacks, and of the configuration and location of the gaskets.
Several seal solutions are presently being studied, i.e.: cements or ceramic adhesives, glass or vitroceramic gaskets, metal compressive gaskets, mica compressive gaskets, brazed gaskets and mixed solutions resorting to several of these techniques.
These gaskets should give the possibility of providing the seals between the cathode chamber and the outside, between the anode chamber and the outside, and between both chambers, and thereby avoid gas leaks between both chambers and towards the outside.
As this has already been specified above, we are more particularly interested in glass gaskets herein.
The glasses used for these gaskets may either be made of a simple glass, or made of a crystallizable glass also called a vitroceramic, or further made of a mixture of both of these glasses, or further made of a simple glass to which ceramic particles are added.
Most glasses used for these gaskets are generally found in solid form at the temperature of use, i.e. generally between 600° C. and 1,000° C., notably between 700° C. and 900° C., for example 850° C. These gaskets are described as <<hard>> gaskets.
The main constraint to be observed in this situation is to formulate a gasket having a thermal expansion coefficient <<TEC >>, adapted to the other elements of the junction, notably to the parts made of ceramics and to the metal parts.
As regards simple glasses, SiO2—CaO—B2O3—Al2O3 compositions are studied in document [1], BaO—Al2O3—SiO2 (BAS) compositions are described in document [2] and in document [3], and finally Li2O—Al2O3—SiO2 compositions are mentioned in document [4], but it is difficult with these compositions to attain TECs adapted to the junctions.
Vitroceramic glasses (or more simply vitroceramics) are, as for them, generally shown as being more chemically and mechanically resistant by controlling the crystallization of the glass with nucleating agents and with particular heat treatments.
These vitroceramic glasses have particular compositions, which ensures that these glasses are amorphous when they are melted during their elaboration, but they then partly or totally crystallize after a suitable heat treatment.
The sought goal is to form crystalline phases which on average will give the gasket a high thermal expansion coefficient, so as to be able to accommodate expansions during thermal cycling operations to which the ceramic/gasket/metal or metal/gasket/metal assemblies will have to be subjected.
The parameters to be controlled for these vitroceramic glasses are the formulation of the glass and the thermal cycles in order to manage to form the crystalline phase(s) having the sought properties.
Many vitroceramic glasses compositions have already been described.
Thus, vitroceramic glass compositions comprising SiO2, BaO, and Al2O3, and optionally SrO, CaO, K2O or B2O3 are mentioned in documents [6] and [7].
Vitroceramic glasses compositions based on SiO2, BaO, CaO, Al2O3, B2O3, and optionally La2O3 are the subject of document [8].
Vitroceramic glasses compositions comprising the SiO2, Al2O3, B2O3, La2O3, and SrO oxides are described in document [9].
Vitroceramic glasses compositions comprising BaO, CaO, Al2O3, SiO2 oxides, and optionally ZnO, PbO, B2O3 or V2O5 are mentioned in document [10]. Such compositions are compositions said to be of the BCAS family or of the BCAS type [10].
Vitroceramic glasses compositions comprising BaO, B2O3, Al2O3, SiO2 oxides are the subject of document [11]. Such compositions are compositions said to be of the BAS family, or of the BAS type.
Vitroceramic glasses compositions comprising SiO2, Al2O3, B2O3, MgO oxides are described in document [12].
Vitroceramic glasses compositions comprising SiO2, BaO, B2O3, Al2O3 oxides, and optionally Ta2O5, SrO, CaO, MgO, Y2O3, La2O3 oxides are mentioned in document [13].
Vitroceramic glasses compositions comprising SiO2, BaO, ZnO, B2O3, MgO oxides are described in document [14].
Vitroceramic glasses compositions comprising SiO2, Al2O3, CaO oxides and optionally SrO, BaO, MgO, ZnO, Nb2O5, Ta2O5, K2O, GeO2, and La2O5 oxides are described in document [15].
Vitroceramic glasses compositions comprising SiO2, CaO, BaO, Al2O3 oxides and optionally SrO oxide are mentioned in document [16].
Compositions of vitroceramic glasses comprising SiO2, CaO, MgO, Al2O3 oxides are the subject of document [17].
However, the development of formulations and of heat treatments for vitroceramic glasses remains delicate since the junction material changes over time, with modification of the crystalline phases and because of the creation of interfaces between the materials in contact. Industrial development of this type of vitroceramic glasses therefore remains complex.
Most vitroceramic glasses compositions mentioned above therefore have drawbacks, in particular related to the fact that they interact with the substrates with which they are in contact, which then causes degradations reducing the performances of the systems such as fuel cells and high temperature electrolyzers, in which these compositions are applied.
The addition of ceramic particles of different sizes and shapes to simple glasses gives the possibility of controlling and adjusting the viscosity and the TEC of the sealing material [18, 19]. The delicate point lies in the presence of a glassy phase in a large amount which may pose corrosion or evaporation problems at high temperature.
In addition to the <<hard>> gaskets described above which appear in solid form at the operating temperature, SrO-La2O3—Al2O3—B2O3—SiO2 compositions, with which a fluid state of the glass may be obtained at operating temperatures, are disclosed in document [5]. These compositions give the possibility of accommodating large TEC differences, but the formulations developed in this document do not prove to be sufficiently resistant from a mechanical point of view, exactly because of this too large fluidity of the glass, so as to be able to maintain the seal against the imposed pressure differences.
It emerges from the foregoing that presently there does not exist any glass composition and more particularly any glass composition belonging to the family of vitroceramics giving satisfaction for a use in seal gaskets for devices, apparatuses operating at high temperatures such as high temperature electrolyzers or high temperature fuel cells.
Therefore there exists a need for a glass composition, and more particularly for a glass composition belonging to the family of vitroceramics, with which it is possible to create a solid gasket, seal, suitable for the seal function for high temperature application for example in high temperature electrolyzers or high temperature fuel cells.
In other words, there exists a need for a glass composition, and more particularly for a glass composition of the vitroceramic type which gives a chemically and mechanically resistant gasket, seal, notably having mechanical properties allowing it to be adapted to the sometimes very different TECs of the very diverse materials, to be assembled with which it is in contact, such as metals and/or ceramics, notably during heating and cooling cycles.
In other words, and more specifically, there exists a need for a glass composition, and more particularly for a glass composition of the vitroceramic type (also called a vitroceramic composition) which advantageously has a high expansion coefficient allowing it to accommodate the different TECs of the materials with which it is found in contact, notably during heating and cooling cycles.
There also exists a need for a glass composition and more particularly for a glass composition of the vitroceramic type which is not subject to corrosion or evaporation phenomena at high temperatures.
There further and especially exists a need for such a glass composition, which does not have or has only very little interactions with the materials to be assembled. In other words, the interactions of the glass composition with materials of the various substrates with which it comes into contact, notably when this composition is applied as a gasket for assembling these substrates, has to be limited, or even nonexistent.
The glass composition should have the properties listed above, notably absence or quasi-absence of interaction with the material(s) of the substrate(s) and a TEC adapted to the material(s) of the substrate(s) regardless of this(these) material(s), whether these are ceramics such as zirconia stabilized with yttrium or YSZ, or Macor®, and/or metals or alloys such as iron-based alloys such as Crofer®, or F18TNb®, or nickel-based alloys such as Haynes® 230.
Finally, there exists a need for a glass composition which may be prepared in a reliable, easy and reproducible way without notably resorting to complex thermal cycles.
Finally there exists a need for such a glass composition, all the properties of which remain stable over time, in particular under conditions of high temperatures.